1. Field of the Invention
The present invention relates to an image recognition apparatus installed in a vehicle or the like, for recognizing an image of a particular recognition target included in image information taken on a road, and to a method of recognizing such an image.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 11-213155 (pages 2 to 3, FIG. 1) discloses a technique to recognize a particular recognition target, based on image information of an image taken on a road. That is, Japanese Unexamined Patent Application Publication No. 11-213155 discloses an apparatus adapted to recognize a boundary line (a lane mark) on the surface of a road, based on image information taken on the road. The apparatus is installed in a vehicle that runs on a road, and the apparatus includes image pickup means for taking an image of a road ahead of the vehicle, and recognition means for recognizing a boundary line on the surface of the road, based on the image information taken by the image pickup means. The recognition means recognizes the boundary line by using a string model whose shape is modified depending on an image feature value of the boundary line included in the image taken by the image pickup means. As for the image feature value of the boundary line, brightness or chromaticity of each pixel is used.
In this apparatus, binarization of image information and expansion from a point to line are not performed to prevent instability in recognition caused by a variation in an external environment, which can occur when the binarization or the expansion is performed, thereby achieving high recognition accuracy for the boundary line.
In the conventional techniques described above, the recognition accuracy for a recognition target such as a boundary line is increased by improving a recognition algorithm based on image information taken by a camera or the like. However, in such a conventional technique, because only image information is used in the recognition process, if a correct image feature value cannot be acquired from image information, a significant reduction in the recognition accuracy occurs.
For example, when image information includes an image of an object having a feature value that is so similar to that of a recognition target to be recognized that it is difficult to distinguish the recognition target from the similar object, or when, depending on the shooting condition in terms of weather or brightness or depending on the recognition target, the condition of objects in the vicinity of the recognition target, the type of the recognition target, there is no significant difference in terms of the luminance or color between the recognition target to be recognized and other objects in the vicinity of the recognition target, it is difficult to correctly recognize the recognition target.